Forged flat top pistons, which brand do you consider the best?

JE

What are you using them for? You probably don't need a JE piston unless you are going with big boost/big nitrous/big rpm - a SRP (JE/SRP, but different than a JE piston - 4032 for the SRP, 2618 for the JE).

The SRP's are cheaper as they are more of an "off the shelf" piston, are probably stronger than you are going to need (unless you meet one of the above categories), and are a bit less prone to piston slap/can run tighter clearances.

Not saying the other brands out there are bad, but the above 2 are definitley among if not the best for your application out there.